Seeking advice on birch bark wrapped sinew backed bow

[Robert Pougnier]

I recently finished my first sinew backed bow. I was working on a  HHB static recurve and a splinter lifted during mid-tillering stages. I decided to save the bow by backing it with a heavy layer of sinew.

I let it cure 20 days, finished tillering it, and shot it in about 75X . The tiller was great and I was very happy with it.

I then decided to wrap it with birch bark in the style of some ottoman bows I'd seen pictures of. I was still very happy with it until I strung it again 10 days after wrapping with bark). It immediately displayed some unevenness in the tiller. The lower limb is now a fair bit stiffer in the near handle portion of the bow.

I've tried gently warming the back of the stiff limb and flexing it while strung, which seems to work until i start bringing it in to full draw. The uneven tiller shows up again.

Does anyone have advice on ways to fix this? Should i just scrape the bark off the belly and retiller it to a lighter weight?

The limbs both have some minor twisting, which exaggerates the unnevenness but it is still fairly obvious. I'm concerned about durability more than any cosmetic issue.

Thanks for looking!

Here are the specs and some photos. 66" NTN, 1 3/4" after fades and through midlimb. 55# at 28". HHB with sinew backing. Purple heart side lams at the tips. The birch bark is thinner than paper and glued with diluted TB3. My guess is moisture creeped into the sinew layer when the birch bark was curing. I've been reluctant to shoot it since.

First photo is braced. 2nd drawn to 20, 3rd drawn to 28.

[Robert Pougnier]

a few more photos of the detail.

[Pat B]

First do you live in a humid climate? The sinew will continue to dry over a long period of time and the birch bark wrap will slow that down some. What glue did you use for the sinew and bark wrap? I think I would try to remove the birch bark wrap, let the sinew dry longer then retiller the bow. If you want to reapply the birch bark use it as a backing instead of a wrap. As the sinew continues to dry the bow will get stronger and the tiller may continue to go off.

[Robert Pougnier]

First do you live in a humid climate? The sinew will continue to dry over a long period of time and the birch bark wrap will slow that down some. What glue did you use for the sinew and bark wrap? I think I would try to remove the birch bark wrap, let the sinew dry longer then retiller the bow. If you want to reapply the birch bark use it as a backing instead of a wrap. As the sinew continues to dry the bow will get stronger and the tiller may continue to go off.

I live in a moderately humid climate, northern Vermont. I'm down in SC for a few months right now and keeping the indoor RH at roughly 48%. I think maybe I rushed the curing process too much though. I did not account for how much the birch bark would slow the drying of the glue I used to apply it. Titebond 3 was used both to apply the sinew and the bark. I should have used hide glue but I was not so much trying to increase performance as much as just saving the bow (I hate wasting materials I harvest). I assumed,  probably wrong, that TB3 would be more stable and cure quicker.

That sounds like a reasonable plan though, and i'm in no rush to shoot it until I've fixed it. I'll get to tinkering with it and give an update soon.

Thanks Pat!

Thank you!

[JW_Halverson]

I think we may have lit on your problem. Some of the moisture migrates direct to the atmosphere from the back of the bow and some migrates INTO THE WOOD! Think about how long it takes to get the last moisture seasoned out of a piece of wood. And especially if you have a finish over the belly of the wood to help block the migration of that moisture!  There is a reason why those old bowmakers building sinew composite bows took their sweet time. Dry to the touch and cured are two completely different critters.

I am going to counsel putting that bow up somewhere out of the way but where air regularly circulates. It's not a head of lettuce that's wilt on ya, or an eg that's gonna go off. Think of it as a fine wine that needs to mature and come into its own. I'm gonna say give it six months.